1. Field of the Invention
This invention relates generally to television receivers, and is directed more particularly to a detecting circuit in television receiver in which a sound signal of high quality can be derived from a received television signal.
2. Description of the Prior Art
In FIGS. 1 and 2 which are block diagrams showing prior art color television receivers, 1 designates an antenna for receiving a broadcast color television signal which is supplied through a high frequency amplifier 2 to a mixer circuit 3 which is also supplied with a signal from a local oscillator 4. A resulting frequency converted signal from the mixer 3 is supplied through a band pass filter 5 as a video intermediate frequency (VIF) signal to a VIF amplifier 6. Here, the band pass filter 5 has predetermined response peak at the video carrier frequency of 58.75 MH.sub.z, at the sound carrier frequency of 54.25 MH.sub.z, and at the carriers of the video and sound signals in the adjacent channels on either side.
In the prior art example shown in FIG. 1, the VIF signal from the VIF amplifier 6 is supplied through a sound trap circuit 7 to a video detector 8, which is usually a diode detector. The VIF signal and sound signal component are supplied from an intermediate stage of the sound trap circuit 7 to a band pass filter 9 which selectively passes these signals to a detector 10, which is also usually a diode detector. A video signal including the carrier color signal is provided at an output terminal 11 of the video detector 8 and a sound intermediate frequency (SIF) signal is provided at an output terminal 12 of the detector 10. To obtain the sound signal the SIF signal is applied from the output terminal 12 to a tuning circuit of 4.5 MH.sub.z (not shown).
In the prior art example of FIG. 2 a synchronous detector is used, as the video detector stage. In this example, the VIF signal from the VIF amplifier 6 is supplied to a band pass filter 13 and then via a limiter 14 as a carrier of 58.75 MH.sub.z to one input of a multiplier circuit 15. The VIF signal from the VIF amplifier 6 is supplied directly to another input of the multiplier 15. One output terminal 11 of the multiplier 15 provides the video signal and another outut terminal 12 of the multiplier 15 provides the SIF signal via a tuning circuit of 4.5 MH.sub.z (not shown) to the output terminal 12.
If the characteristic of the band pass filter 5 is ignored, as is done here for the sake of brevity, the amplitude-to-frequency characteristic of the output from the detector 10 in FIG. 1 or from the multiplier 15 in FIG. 2 is flat, as shown in the graph of FIG. 3. Therefore, in both the prior art examples of FIGS. 1 and 2, in order to separate out the SIF signal from any video signal component mixed therewith, and to avoid mutual interference (or so-called "buzz") between the video signal and the SIF signal, it is necessary to include a band pass filter having a sharp characteristic with a 4.5 MH.sub.z center frequency as shown in the graph of FIG. 3 by the broken line, connected to the output terminal 12.